In recent years, an electronic apparatus, such as a personal digital assistant or a communication terminal, impressively achieves high performance and is reduced in size. A personal digital assistant or a communication terminal is embedded with a resonator filter. The reduction in the size of the personal digital assistant or the like increases demands for the reduction in the size of the resonator filter. Accordingly, a dielectric resonator is increasingly used.
As the dielectric resonator, Patent Document 1 describes a multiple-mode dielectric resonator. In this multiple-mode dielectric resonator, a dielectric resonant element is arranged in a cavity resonator, and a metal screw is provided in the cavity resonator toward the dielectric resonant element so as to generate a resonant coupling mode. Thus, this multiple-mode dielectric resonator copes with a plurality of frequencies. In such a dielectric resonator, however, there is a problem in that the metal screw in the resonator causes an increase in the transmission loss of electrical signals for resonance.
In order to solve the above-described problem, Patent Document 2 describes a multiple-mode dielectric resonator in which a columnar opening is formed with respect to a dielectric resonant element so as to perform a resonant coupling mode. Thus, this multiple-mode dielectric resonator copes with a plurality of frequencies. In such a dielectric resonator, however, processing, such as cutting or the like, is needed, which results in an increase in manufacturing costs. Further, a sufficient resonant coupling mode may not be generated. As a result, a practical multiple-mode resonator which copes with a plurality of frequencies may not be realized.    Patent Document 1: Japanese Patent Unexamined Publication No. S57-194603-A    Patent Document 2: Japanese Patent Unexamined Publication No. S62-204601-A